The present invention relates to an optical device for use in measuring the proximity or nearness of a surface. The invention also relates to the application of such a device to the plotting or surveying of the profile or contour of a surface.
Such devices are used in numerous fields and in particular for controlling robots, which automatically perform certain tasks. They make it possible to obtain instantaneous information on the relative position of a member relative to a surface and, if appropriate, the profile of the surface. The movements of the robot, as well as the function fulfilled by the robot, can consequently be automatically adapted at all times to the working conditions which can evolve over a period of time.
When they are used for plotting the profile of a surface, such optical devices are advantageously used in adoptive welding robots, as well as in computer-assisted welding. Thus, these two procedures make it necessary to know the spatial position and also certain geometrical parameters of the welded joint. This knowledge is obtained as a result of proximity measuring devices and profile plotting devices.
The prior art optical surface proximity devices are of two different types.
A first group of known optical sensors utilizes the so-called triangulation principle. According to this principle, a light beam is transmitted obliquely towards the surface to be detected, in order to form a light spot thereon. The beam reflected and/or diffused by the surface of focused, in order to form the image of the spot on a position finding detector. The latter can in particular comprise a camera with a charge transfer device. Such an optical sensor is marketed by the Swedish company SELCOM under the trade name OPTOCATOR.
Like all optical devices, said device requires no material contact with the surface to be detected. Moreover, its operating distance with respect to the surface (called the flight height) is satisfactory. Finally, this optical sensor is protected against electrical interference, which is highly prejudicial in certain applications and particularly in the case of welding.
However, triangulation sensors also suffer from the disadvantage of using a complex, costly position finding detector. In addition, they have a relatively limited passband.
There are also infrared optical sensors, in which the infrared radiation emitted by a light-emitting diode is reflected by the surface onto a detector, such as a photodiode or phototransistor. The detector supplies an analog signal representative of the light flux reflected by the target and in particular the distance between the sensor and the target. These infrared sensors are simpler and less expensive than triangulation sensors, whilst they also have a wider passband.
However, the responses of these sensors vary with the surface reflection coefficient and with the angle of incidence of the incident beam on said surface. Thus, they are difficult to use when the surface state of the part and/or its orientation relative to the sensor vary, which is generally the case in robotics.
The present invention specifically relates to an optical surface proximity measuring device similar to the second type of existing optical sensors described hereinbefore but which, contrary to the latter, makes it possible to measure the distance separating the sensor from the facing surface independently of the reflection coefficient of said surface and the angle of incidence. As in the case of all optoelectronic sensors, the sensor according to the invention also has the advantage, compared with optical triangulation sensors, of replacing a complex, costly detector by a bottom range optical detector, whilst also increasing the passband.